This invention relates to channel induction furnaces such as are used for melting metals.
The invention applies to furnaces for melting all types of metals but is particularly applicable to metals having high electrical conductivity such as aluminium and copper. For such metals high current densities are required to produce a high power input. If the channel cross sectional dimensions are comparable with the depth of penetration of the induced current then the interaction of this current with the net magnetic induction produces electromagnetic forces directed away from the walls of the channel. This squeezing action on the metal, which is referred to as an electromagnetic pinch, produces an increase in static pressure towards the centre of the channel relative to that at the wall. If the current density is not too high, this increase in static pressure is balanced by the static head of the molten metal above the channel. However, there will be some limiting current density, and corresponding maximum power input, for which the increase in static pressure exceeds the head of liquid metal and the metal is forced away from the walls of the channel. As the current is now concentrated into a conductor of smaller cross sectional area, the pinch forces increase causing a still greater contraction of the conducting area. For sufficiently large power inputs, a break occurs in the metal thus interrupting the current. Without a current there are no electromagnetic forces and the metal flows back under the influence of gravity to re-establish the current path. The cycle then restarts leading to a repetitive interruption of the electrical power. The power input for which pinching occurs will be lowest for metals of high electrical conductivity and low density, such as aluminium.
The pinch effect and the limitations it imposes on power input are well known to those familiar with channel induction furnaces. It is also known that the pinching effect can be avoided by making the radial width, W, of the channel considerably greater than the depth .delta. of penetration of the induced current. The radial width, W, is measured radially outward from the axis of the induction coil in the plane at right angles to the coil axis and in a direction normal to the axis of the channel at the point of measurement. Although this arrangement avoids the pinch effect, cavitation phenomena (described in more detail below) will occur for sufficiently high current densities.